A conventional CATV converter, as illustrated in FIG. 7, comprises:
(1) an input terminal 57A, PA1 (2) a first local oscillator 51, PA1 (3) a first mixer 52, where an input signal to the input terminal 57A is fed to a first terminal of the first mixer 52, and an output from the first local oscillator 51 is also fed to a second terminal thereof, PA1 (4) an intermediate frequency filter 53 to which an output of the first mixer 52 is fed, PA1 (5) a second local oscillator 54, PA1 (6) a second mixer 55, where an output of the intermediate frequency filter 53 is fed to a first terminal of the second mixer 55, and an output of the second local oscillator 54 is also fed to a second terminal thereof, PA1 (7) an output circuit 56 coupled with the output terminal of the second mixer 55, and PA1 (8) an output terminal 57B coupled with an output terminal of the output circuit 56. PA1 (1) a first filter coupled with an output terminal of a second mixer, where a first filter attenuates frequencies approximately equal to an audio carrier of an upper adjacent channel and video carrier of a lower adjacent channel, PA1 (2) a first amplifier coupled with an output terminal of the first filter, PA1 (3) a second filter coupled with an output terminal of an first amplifier, where a second filter attenuates a video carrier of the upper adjacent channel and an audio carrier of the lower adjacent carrier, PA1 (4) a second amplifier disposed between the second filter and the output terminal, and has the following function: when an input signal fed to the input terminal is within a predetermined frequency range, the input and output terminals of the second filter are short-circuited because of a high frequency of the input signal.
The output circuit 56 comprises the following elements coupled in series in this order: a first filter 58, a first amplifier 59, a second filter 60, and a second amplifier 61.
An operation of the above conventional CATV is explained here:
A signal fed into the input terminal 57A is mixed in the first mixer 52 whereby being converted to a first intermediate frequency. The output thereof is fed to the intermediate filter 53, and only an intermediate frequency bandwidth is tapped off. The output thereof is fed to the second mixer 55, whereby being converted to the second intermediate frequency before being tapped off. The output thereof is fed to the output circuit 56, and then tapped off from the output terminal 57B. In the output circuit 56, a signal tapped off from the second mixer 55 is fed to the first filter 58, where a video carrier of an upper adjacent channel and an audio carrier of a lower channel are attenuated and then tapped off. The output thereof is fed to the first amplifier 59 and amplified, then tapped to the second filter 60, where an audio carrier of the upper adjacent channel and a video carrier of the lower channel are attenuated and then tapped off. Further the output thereof is fed to the second amplifier 61 to be amplified, then tapped off.
In this conventional structure, a trap is provided nearby the received signal bandwidth because of the first filter 58. Therefore, in the system which handles digital and analog signals, when receiving compressed digital signals, original signals cannot be reproduced due to deterioration of a differential phase or differential gain in the receiving signal bandwidth.